Compression of a cryogenic medium

ABSTRACT

A method for compressing a cryogenic medium, in particular hydrogen, oxygen, nitrogen or argon is described. The cryogenic medium is compressed in two compressor stages from an initial pressure to a final pressure by way of an intermediate pressure, such that the first compressor stage is designed as a cryogenic compressor stage. The cryogenic medium is advantageously compressed to a pressure between 30 and 70 bar in the first compressor stage and in the second compressor stage is compressed to the desired final pressure by means of a hot gas compression.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from German Patent ApplicationDE102012003446.6 filed Feb. 21, 2012.

BACKGROUND OF THE INVENTION

The invention relates to a method for compression of a cryogenic medium.

Generic methods for compressing cryogenic media, such as hydrogen,oxygen, nitrogen or argon, for example, are known from the prior art.

The term “cryogenic medium” is usually understood to refer to aliquefied cryogenic gas which is at a comparatively low temperature. Forexample, the temperature of cryogenic hydrogen is usually between −253°C. and −245° C.

Generic methods for compression of cryogenic media may be used infilling and refilling storage tanks. For example, various fillingmethods are used to fill hydrogen storage tanks installed in motorvehicles:

Pressure compensation methods: Several gas buffer storage devices havingdifferent pressure levels are filled from a supply system; this may be astationary storage tank or a pipeline, by means of a compressor or acryopump. In the case of refueling a motor vehicle, the compressedhydrogen is filled into the storage tank in the vehicle from the gasbuffer storage devices by pressure compensation until reaching the finalrefueling pressure.

Booster Method: In this case, hydrogen from a supply system iscompressed by means of a high-performance compressor and thentransferred directly into the automotive storage tank.

Combination of pressure compensation and booster methods: Here there isfirst a partial filling of the vehicle storage tank that is to be filledfrom the gas buffer storage devices by means of pressure compensationbefore being filled to the final pressure by the booster method.

In addition, there are refueling methods in which the hydrogen iscompressed by means of cryogenic compression to 700 bar with temperaturecompensation up to maximum 875 bar, such that the boil-off gas formed incryogenic compression is sent to the automotive storage tank in a firstrefueling step. A cryogenic compression of liquid hydrogen tosupercritical gaseous hydrogen takes place at entrance temperaturesbetween −253° C. and −245° C. Compression of the boil-off gas formed incryogenic compression requires so-called hot gas compressors, by meansof which gas at ambient temperatures is compressed at ambienttemperatures, which are understood to be in a temperature range between−20° C. and 40° C. However, such hot gas compressors are comparativelyexpensive.

Furthermore, this requires a compressor system of at least two to threestages because of the required compression ratio, but such a systemwould be unfavorable from an energy standpoint. The hydrogen must alsobe warmed by means of an ambient air evaporator before being fed intothe hot gas compressor and therefore it loses the advantage of the highdensity as a cryogenic gas. So far there are not any cryogeniccompression systems that could be used for compression of the boil-offgas to the required pressure level of 400 to 500 bar. However, pressuresof 400 to 500 bar are necessary to be able to utilize the boil-off gasfor refueling the vehicle according to the current refueling technique,in which the final refueling pressures are between 700 and 800 bar.

The object of the present invention is to provide a method forcompression of a cryogenic medium, in particular hydrogen that willavoid the aforementioned disadvantages.

SUMMARY OF THE INVENTION

To solve this problem, a method for compressing a cryogenic medium isproposed, characterized in that the cryogenic medium is compressed intwo compressor stages from an initial pressure to a final pressure byway of an intermediate pressure, and the first compressor stage isdesigned as a cryogenic compressor stage.

The term “cryogenic compressor stage” and/or “cryogenic compression” isto be understood below as a compression process in which a cryogenicliquid medium is converted into a compressed supercritical gas and inwhich the entrance temperature of the cryogenic medium is below −70° C.Cryogenic compression of liquid hydrogen or other liquefied cryogenicgases is usually performed by using cryogenic piston pumps. Thecryogenic medium to be compressed here enters the piston as a liquid andis ejected as a supercritical gas.

According to the invention, the cryogenic medium is then compressed intwo compressor stages from an initial pressure to a final pressure byway of an intermediate pressure, such that the first compressor stage isdesigned as a cryogenic compressor stage. In this process, the cryogenicmedium having an initial pressure between 1 and 3 bar is preferablycompressed to a pressure between 30 and 70 bar in the cryogeniccompressor stage.

The method according to the present invention for compressing acryogenic medium is advantageously implemented in a combined compressorin which the first cryogenic compressor stage compresses the cryogenicmedium from the initial pressure to the desired intermediate pressure.This cryogenic compression makes use of the fact that the medium to becompressed is present with a high density and therefore the compressioncylinder can be designed to be relatively small. As a result, therequired driving force for the compression is low, so the result is anenergetically favorable compression process.

In the first compressor stage the medium is brought to a highertemperature so that the medium in the second compressor stage can becompressed to the final pressure by means of a hot compression process.Because of the pre-compression in the cryogenic compressor stage, thecompression space required for the second compressor stage can bedesigned to be relatively small.

It is proposed that to further refine the method according to theinvention for compressing a cryogenic medium, the two compressor stagesshall be implemented in piston compressors which are driven via a shareddrive. For example, an electric motor with a double gear for operationof two compressor stages may be used as the shared drive.

In addition, according to an advantageous embodiment of the methodaccording to the invention for compression of a cryogenic medium, onesingle-piston compressor is used for each compressor stage, such thatthe two single-piston compressors are advantageously operated in as areciprocating compressor.

In the case of an optimum design of the compression ratio in the twosingle-piston compressors, i.e., compressor stages, these may beoperated as a reciprocating compressor by means of a shared drive. Indoing so, the piston here compresses the cryogenic medium first in thefirst compressor stage. At the same time the piston of the secondcompressor stage is in the return stroke, and in doing so, draws thepre-compressed medium out of the first compressor stage. When the secondstage is compressed, the first compressor stage is in the return strokeand again draws in cryogenic medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The figure is a schematic of a method for compressing a cryogenic mediumaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention for compressing a cryogenic mediumwill be explained in greater detail below on the basis of the exemplaryembodiment illustrated in the figure.

The pump arrangement shown in the figure consists of two single-pistoncompressors, each having a compression space V1 or V2, a piston K1 or K2driven by a piston rod and a working space A1 or A2, which is requiredto drive the pistons. One intake valve a or c and one outlet valve b ord is assigned to each compression space V1 and V2.

The medium to be compressed is sent to the first compressor stage and/orto the first compression space V1 through line 1. This is a speciallyinsulated, preferably vacuum-insulated line, which reduces the unwantedheat input to the medium to be compressed to a minimum. The medium to becompressed then flows into the compression space V1 with the intakevalve a open. The inflowing medium usually is at a pressure between 1and 3 bar. Cryogenic compression to a pressure between 30 and 70 bartakes place in the compression space V1. Next the compressed medium isconveyed into an equalizing tank 2 arranged between the two compressionspaces V1 and V2 with the outlet valve b open. The medium flows out ofthe equalizing tank when intake valve c is opened there into the secondcompressor stage V2 in which compression to the desired final pressuretakes place. With the outlet valve d open, the medium compressed to thefinal pressure is removed through a (high-pressure) line 3.

The figure shows the moment when the piston K1 in the compression spaceV1 is at top dead center and thus at the end of the intake stroke, whilethe piston K2 in the compression space V2 is at bottom dead center sothe compression stroke is ended.

The drive for the two reciprocating pistons K1 and K2 is provided bymeans of a hydraulic pump P, which pumps the hydraulic fluid in theworking spaces A1 and A2 through the lines 4 through 7, so that itresults in an up-and-down movement of the pistons K1 and K2. Instead ofthe aforementioned hydraulic pump P, an electric motor with areciprocating double gear may also be used, for example.

The method according to the invention for compression of a cryogenicmedium has a number of advantages in comparison with the compressionmethods that are part of the prior art:

-   -   favorable compression energetically and in terms of plant        technology because only comparatively small compression spaces        and thus low drive powers are necessary,    -   no preheating of the cryogenic medium is necessary in contrast        with the hot gas compression described above; furthermore there        is no need for an intermediate cooler between the two compressor        stages,    -   a high final pressure can be achieved at a nominal performance,    -   despite two different compression systems (“cryogenic” and        “hot”), only one common drive system is required for optimal        design,    -   with respect to design space, complexity and energy efficiency,        the method according to the invention for compression of a        cryogenic medium constitutes a significant improvement in        comparison with the compression methods of the prior art.

What we claim is:
 1. A method for compressing a cryogenic medium,characterized in that the cryogenic medium is compressed in twocompressor stages from an initial pressure to a final pressure by way ofan intermediate pressure, and a first compressor stage is designed as acryogenic compressor stage.
 2. The method according to claim 1,characterized in that the cryogenic medium is compressed to a pressurebetween 30 and 70 bar in the first compressor stage.
 3. The methodaccording to claim 1, characterized in that the two compressor stagesare implemented in piston compressors and are driven by means of ashared drive.
 4. The method according to claim 3, wherein asingle-piston compressor is used per compressor stage, characterized inthat the two single-piston compressors are operated as a reciprocatingcompressor.
 5. The method according to claim 1, characterized in thatthe cryogenic medium is selected from the group consisting of hydrogen,oxygen, nitrogen and argon.